1. Field of the Invention
The present invention relates to a copying optical system comprising a lens system in which the change of focusing ability caused by the change of temperature is well compensated.
2. Description of the Prior Art
It is well-known that the refractive index of optical glass is variable with temperature. Accordingly, a lens system composed of optical glass varies in its refractive index with temperature, thereby changing the focal length of the total lens system.
Generally speaking, in the case of a photographing lens system, the possible displacement of focus due to temperature change is relatively small. In addition, for a photographing lens system it is generally possible to suitably adjust the focus. Therefore, in the case of a photographing lens system, the problem of change of focal length with temperature is not critical and has been left ignored in many cases.
In contrast, in the case of lenses generally used in copying machines, such change of focal length caused by temperature change brings forth a critical problem. In the case of a copying lens system, the magnification at which an image of an original is formed, is mainly 1X or near it. This means that the displacement of focus is not in linear proportion to the change of focal length of the lens system but four times as much as the change of focal length. Further, when the focus has once been adjusted in the course of manufacture of the lens system, the latter works as a fixed lens. If there occurs a large displacement of focus in such a fixed copying lens, the quality of copies then produced varies from time to time with temperature change. Such variation of copy quality will bring forth a difficult problem in quality control.
Hitherto, in most copying machines there have been used lenses of the orthometer type. In the case of orthometer type lenses conventionally designed, the variation of focal length caused by the change of refractive index with temperature change is not so large.
In contrast, in the case of the above mentioned positive-negative complete symmetrical lens, the focal length thereof is apt to greatly change with change of temperature.